The present invention relates to a method for generating a modulated television signal with improved characteristics.
The television transmission mode that uses two carriers, one for audio and one for video, is known in the art.
As regards the video part, the video signal modulates a video carrier with positive or negative modulation and is then shaped with a partially suppressed sideband. This type of modulation is termed vestigial modulation (VSB--Vestigial Sideband), and with respect to conventional amplitude modulation it has the advantages of reducing the occupied band without appreciably degrading picture quality, requiring less power to transmit the information, in addition to a good response to low frequencies.
For the audio part, the audio signal frequency modulates the carrier.
In old transmitters, modulation was performed by amplifying the carrier to the maximum power level and then modulating it. Filtering was performed at high power with the use of very expensive and bulky power filters that were highly unstable and therefore required periodic adjustments and were highly temperature-sensitive, requiring special conditioning systems.
Currently, the video part of the signal is generated with analog methods. The video carrier is modulated, at an intermediate frequency (IF), by the video signal, generating a signal with two sidebands, and is then filtered in order to obtain the desired shaping of the sidebands. Filtering is achieved with analog filters of the SAW (Surface Acoustic Wave) type, which must be produced differently according to the television standard that is used. A drawback of this solution is the fact that the generated television signal has limited characteristics in terms of quality, linearity, flexibility, and stability, which are a consequence of the analog-type processing to which it has been subjected. Another drawback of this solution is the fact that it requires many level adjustments for white, black, sync signals, and other parameters that are necessary for linear power amplification. Furthermore, analog generation of the video signal has the drawback that if one wishes to transmit a digital signal, which is the future of television development, it requires a subsequent conversion from analog to digital, with further adjustments. Finally, the generated television signal does not have particularly good characteristics in terms of quality, linearity, and stability.